Variably timed intake and exhaust valves are used in a variety of engines, notably compression ignition internal combustion engines. Conventional engine systems typically utilize a rotating cam configured to control opening and/or closing of engine valves at desired times during an engine cycle. Variably timed valves provide some flexibility in opening or closing timing in certain engine systems via actuators configured to control valve position separately from cam rotation. Such systems can thus provide enhanced control over cylinder conditions during an engine cycle, enabling emissions reduction and other strategies such as engine braking. As is well known in the art, the performance characteristics of variable valves can depart from desired specifications under certain conditions.
One known application for variable valve timing relates to holding open intake valves past a time in an engine cycle at which they would normally be permitted to close. In a typical engine cycle, a rotating cam opens an intake valve during at least a portion of an intake stroke in a particular cylinder, then permits the valve to return to a closed position approximately when the corresponding piston reaches a bottom dead center position and begins a compression phase. Variable intake valve actuators may be used to maintain a particular intake valve in an open state, despite the cam ceasing to exert an opening force on the valve.
It is common for variable valves to be actuated via a hydraulic intake valve actuator or piston, separate from the engine cam. As an intake valve moves toward an open position under the influence of a rotating cam, hydraulic fluid is permitted to flow into a control cavity to which a pressure surface of a valve actuator is exposed. A separate control valve may then be used to block fluid draining from the cavity, such that the valve actuator is hydraulically locked to inhibit closing of the intake valve. The intake valve may be held in an open position until such time as the control valve is adjusted to permit draining of fluid from the cavity and consequent returning of the intake valve to a closed position. As alluded to above, however, certain performance characteristics such as the specific timing of valve closing can depart from an optimal timing. Variations in timing can degrade overall engine performance, and emissions quality, and can render certain operating schemes unachievable. In extreme cases, failure of variable valve components can subject the system to potentially damaging cylinder pressures. The control valve responsible for blocking and unblocking fluid draining from the control cavity tends to be a major contributor to valve timing variability and failure in such systems. Evaluation of the performance of individual control valves, however, has heretofore been challenging.
One method of diagnosing problems in an engine having variable valve timing relies upon the use of a pressure sensor coupled with a common rail for supplying actuation fluid to the control cavity of the intake valve actuator. Operation of variable valve actuators in an engine has been shown to be associated with certain changes in pressure in the common rail. Thus, signals from the pressure sensor may be used to indicate changes in rail pressure, or lack of changes, relative to expected changes from variable valve actuator operation. While such systems have performed well over the years, there is room for improvement. Such systems also increase cost and complexity of engine systems.
Another diagnostic strategy for a variable valve mechanism is known from United States Patent Application Publication No. 2005/0126524. In the '524 application, a test strategy is provided wherein misfire is induced in a cylinder of the engine via controlling the state of an engine valve. Proper activation of the valve mechanism may be confirmed by detecting whether a misfire due to keeping the valve open during compression occurs when expected. While the '524 application sets forth one useful strategy, the ability to diagnose valve performance deviations via other means is desirable.
The present disclosure is directed to one or more of the problems or shortcomings set forth above.